Among display devices each of which displays an image based on an inputted video signal, there is a display device which includes a correction circuit for correcting the inputted video signal. In the display device, the correction circuit performs desired correction with respect to the video signal, so as to cause a display panel to display an image based on the video signal.
Here, a liquid crystal display device, which is an example of the foregoing display device, includes a liquid crystal panel which is made of a liquid crystal layer sandwiched by glass substrates, and causes an electrode formed on the glass substrate to apply a voltage to the liquid crystal, thereby displaying the image based on the video signal. In the liquid crystal display device, an electro-optical effect is used so as to realize gradation display. For example, in a TFT (Thin Film Transistor) liquid crystal display device for driving in accordance with a general TN (Twisted Nematic) mode, a polarizing plate and a liquid crystal cell are combined with each other, so as to utilize liquid crystal's optical activity obtained by applying a voltage, thereby varying optical transmittance of the liquid crystal.
As exemplified in FIG. 12, a relationship between the applied voltage and the optical transmittance of the liquid crystal is represented not by a straight line, but by an S-shaped curve. Note that, this example shows a case of a normally black mode in which the transmittance increases as the applied voltage increases. However, also in a case of a normally white mode in which the transmittance adversely decreases as the applied voltage increases, the following description is applicable.
In a transmission type liquid crystal display device in which a light source is disposed on a back side, luminance of a display image is in proportion to the optical transmittance. In the transmission type liquid crystal display device, when an input level of a video signal is shifted to a liquid crystal driving voltage (applied voltage) so as to display an image, a relationship between the applied voltage and the optical transmittance corresponds to a relationship between the input level of the video signal and the luminance of the display image.
That is, in the liquid crystal display device, the relationship (γ characteristic) between the input level of the video signal and the luminance of the display image is represented by an S-shaped curve as in the applied voltage-transmittance curve shown in FIG. 12.
Then, there is a case where the liquid crystal display device is provided with a correction circuit for correcting the γ characteristic for example, so as to exactly display an image based on the inputted video signal.
Further, for example, a television provided with a CRT (Cathode Ray Tube) (hereinafter, the television is referred to as “CRT device) has a γ characteristic completely different from the γ characteristic of the aforementioned liquid crystal display. FIG. 13 shows an example of the γ characteristic of the CRT device. According to FIG. 13, a normalized input level (V) and an output luminance (Y) are related with each other so as to have such an exponential γ characteristic (γ=2.2) that Y=V2.2. Thus, the CRT device is based on a condition under which: an imaging system generates a video signal at 1/γ, and a display system performs γ inversion.
Thus, the liquid crystal display device and the CRT device are different from each other in the output luminance even when the same video signal is inputted, so that a condition under which a halftone image is reproduced greatly differs depending on the difference in the γ characteristic.
Then, in order to realize the display characteristic of the CRT device (in order to replace the CRT device), there is a case where the liquid crystal display device is provided with a correction circuit which realizes the γ characteristic of γ=2.2. Thus, the luminance curve of the liquid crystal is corrected.
Examples of the foregoing liquid crystal display device are disclosed in Japanese Unexamined Patent Publication No. 288468/1997 (Tokukaihei 9-288468)(Publication date: Nov. 4, 1997) and Japanese Unexamined Patent Publication No. 296149/1999 (Tokukaihei 11-296149)(publication date: Oct. 29, 1999).
In these techniques, the applied voltage-transmittance characteristic (input/output characteristic) of the liquid crystal that is shown in FIG. 12 is approximated by a function expression, so as to correct the applied voltage-transmittance characteristic of the liquid crystal on the basis of the approximate expression. More specifically, in Tokukaihei 9-288468, an S-shaped curve indicative of an applied voltage-transmittance characteristic is approximated in accordance with three functions (the line indicative of the applied voltage-transmittance characteristic is divided into three sections in accordance with the voltage level, so as to approximate the sections by functions indicative of curves and a straight line). Further, in Tokukaihei 11-296149, an S-shaped curve indicative of an applied voltage-transmittance characteristic is approximated in accordance with five functions (the line indicative of the applied voltage-transmittance characteristic is divided into five sections in accordance with the voltage level, so as to approximate the sections by using different functions, thereby approximating an S-shaped curve).
However, these arrangements recited in the foregoing publications raise the following problem: since an S-shaped curve indicative of an applied voltage-transmittance characteristic is approximated in accordance with plural functions, the continuity of the plural functions and positions at which the plural functions are connected to each other may be determined, so that it takes trouble and time to perform calculation (since the input voltage is divided into sections so as to approximate the applied voltage-transmittance of the liquid crystal by functions, the continuity of the divisional points may be considered and the points at which the characteristic should be divided may be determined, so that it takes trouble and time to perform calculation).
That is, in the case of dividing the characteristic into sections so as to approximate the characteristic by functions like the foregoing publications, it is necessary to consider the continuity of the divisional points and it is necessary to determine the positions at which the curve should be divided, and it takes much trouble to perform such processes. Further, it is necessary to prepare approximate expressions different from each other so as to correspond to the respective sections, so that a large number of approximate expressions are required. Further, it is necessary to give a large number of parameters to the approximate expressions, so that it takes much time to perform such calculation. Also, such condition raises the problem that the γ correction is inaccurately performed.